1. Field of the invention
This invention relates to a still image video telephone transmission system in which a still picture taken by a video camera is sent to telephone lines during talking over telephone, and more particularly to improvement of a transmitting means and a receiving means so as to correct deteriorated picture signals sent over a transmission path.
2. Description of Related Art
Heretofore video telephones have been developed and widely used in which still picture data are modulated into an audible frequency signal and are sent via telephone lines.
FIG. 20 of the accompanying drawing is a block diagram showing this kind of still image video telephone disclosed in "Details of TTC Standard TV Telephone System," page 31, TV Technique, issued September, 1988.
In FIG. 20, reference numeral 1 designates a still image video telephone transmission system; 2, telephone lines; 3, an external telephone set; 4, a network control unit for inputting/outputting picture information and switching the connection between the video telephone transmission system and the external telephone set 3; 5, a modem circuit for modulating/demodulating the picture data so as to transmit the data via the telephone lines; 6, a demodulation timing recovering circuit for recovering a timing to be used by the modem circuit 5; 7, an A/D converter for converting analog data demodulated by the modem circuit 5 into digital data; 8, a keypad for triggering transmission and various operations; 9, a display for showing a picture of a speaker and that of a counterpart; 10, a video camera; 11, a picture information memory; 12, a picture control circuit to write/read the picture data into/from the picture information memory 11, to output the picture signal to the display 9, and to input the picture signal from the video camera 10; 13, a D/A converter to convert the digital picture signal from the picture control circuit 12 into an analog picture signal and send the analog picture signal to the display 9; 14, an A/D converter to convert the analog picture signal outputted from the video camera 10 into a digital picture signal and to send the digital picture signal to the picture control circuit 12; 15, a central processing unit (hereinafter referred to as CPU) comprising a microcomputer including a ROM and RAM, and processing signals throughout the video telephone transmission system.
FIG. 21 illustrates a method to modulate the picture data by the video telephone transmission system of FIG. 20. This kind of modulation system is based on an amplitude-phase modulation system (hereinafter referred to as AM-PM system) in which amplitude information and phase information are transmitted in combination. The modulation of the picture data having a picture element with luminance "16" will be described with reference to FIG. 21.
As shown in FIG. 21, there are two phases: a first phase of a sine curve type, and a second phase of-sine type different by 180.degree. from the first phase, each of the first and second phases being composed of signals having 8-level amplitudes.
Further, the particular phase and the particular amplitude of one cycle of signal corresponds to respective particular luminancees. In the illustrated example, each phase is eight cycles, i.e., sixteen cycles in total. As is apparent from FIG. 21, the signal "0" of the maximum amplitude of the first phase corresponds to black. The intermediate signals "1" to "14" correspond to gray of respective luminancees. The signal "15" of the maximum amplitude of the second phase corresponds to white.
FIG. 22 is a diagram schematically showing a data format of the conventional system. In FIG. 22, reference numeral 16 designates picture control information including a gain control signal for receiving the picture signal and a picture mode signal; 17, picture data; 18, a dual tone signal for notifying arrival of the picture data 17; and 19, a hardware preamble notifying arrival of the picture data 17. The picture data 17 of the picture element at the upper left side of the screen are sent first and then those for the succeeding picture elements are sent laterally to the right in succession, then are returned to the leftmost side of the line below a first line after they reach the rightmost position, and are sent laterally again.
The operation of the conventional system of FIG. 20 will be described below. First, the put/output signal from the telephone set 3 is broken, and the picture signal from the modem circuit 5 is made ready for outputting the picture signal to the telephone lines 2. Then the CPU outputs the picture control signal (shown in FIG. 18) to the telephone lines 2 through the modem circuit 5. The CPU 15 receives picture data sequentially from the picture information memory 11 via the picture control circuit 12, and sends the picture signal to the modem circuit 5. When the picture data for one picture is transmitted, the network control unit 4 switches the connection to the telephone set.
To receive a still picture, when the modem circuit 5 detects either the dual tone 18 or the hardware preamble in the picture control information 16 shown in FIG. 22, the CPU 15 receives a notice to start receiving information. The CPU 15 switches the network control unit 4 ready for receiving the picture, starting receiving information. The received picture signal is demodulated by the modem circuit 5 and the demodulation timing recovering circuit 6, is converted into a digital signal by the A/D converter 7, and then forwarded to the CPU 15. The picture data for one picture sent to the CPU 15 are sequentially stored in the picture information memory 11 via the picture control circuit 12.
The picture data stored in the picture information memory 11 are sequentially read out via the picture control circuit 12, and are sent to the display 9 so as to have the picture displayed on the screen. Now the speaker can talk while observing the still picture from the other party.
However, in the conventional still image video telephone transmission system, the following disadvantages have been experienced.
(1) When a number of non-particular signals are being sequentially received as picture element signals, clocks outputted by a clock reforming PLL circuit in the modem circuit 5 are displaced by one picture element due to noises or a frequency shift of AM-PM signal over a transmission path, and a picture is received in a slanted condition.
(2) Even if the clock outputted by the carrier recovery circuit PLL in the modem circuit is displaced by 180.degree. due to factors such as noises during the reception of the picture element signals, it is impossible for the receiving side to know this fact. This leads to false recognition of the first and second phases, and makes the received picture be displayed with the black and white colors inverted on the display.
(3) The picture element signals for the right end of the picture and the left end of the next line are transmitted in succession. Generally the signals for adjacent picture elements are less correlated. Therefore, the picture elements remote from each other have very different waveforms due to the large luminance or color difference. And as a result, these signals influence each other, distorting the signal waveforms, and adversely affecting the luminances or colors of the received picture at the right and left sides of the display.